The present invention relates to a method and apparatus for creating a Bragg grating in a waveguide, in particular an apodised Bragg grating.
In this description, reference shall be made to optical fibres, but this reference shall be intended as a matter of example only and not as a limitation, since the technology described is equally applicable also to integrated optical waveguides.
Typically, the optical fibres used for telecommunications are doped with germanium, which induces a photosensitivity property to the UV radiation. To write a Bragg grating in an optical fibre, this property is used to locally modify the refractive index through UV illumination.
As known, an optical fibre Bragg diffraction grating is a portion of fibre which has, in its core, an essentially periodic longitudinal modulation of the refractive index. Said structure has the property of back reflecting the light passing through it in a wavelength band centered around the Bragg wavelength. The Bragg wavelength, as known (for example, from the relation 3.3 of the text xe2x80x9cFiber Bragg Gratingsxe2x80x9d, Andreas Othonos, Kyriacos Kalli, Artech House, Boston/London, 1999), can be expressed as follows:
xcexB=2xc2x7neffxc2x7xcex9xe2x80x83xe2x80x83(1)
where neff is the effective refractive index and xcex9 is the spatial period of the diffraction grating.
Moreover, as known (for example, from the relation 3.4 of the above text xe2x80x9cFiber Bragg Gratingsxe2x80x9d ), the pattern of the refractive index n along axis z of the fibre core can be expressed by the following relation (wherein all of the possible dependencies from variable z are shown):
xe2x80x83n(z)=n0(z)+xcex94n(z) sin(2xcfx80z/xcex9(z))xe2x80x83xe2x80x83(2)
where n0(z) is the mean local value of the refractive index and xcex94n(z) represents the local envelope of the refractive index. The effective refractive index neff is proportional to the mean refractive index n0(z) through a term defining the confinement factor (typically indicated with xcex93) of the fundamental mode of the fibre.
On the basis of the pattern of the refractive index, uniform gratings, so-called xe2x80x9cchirpedxe2x80x9d gratings and apodised gratings are known.
In uniform gratings, the terms n0(z), xcex94n(z) and xcex9(z) are constant, as shown in FIG. 1a, wherein there is represented the typical pattern of the refractive index n (normalised to 1) as a function of the z coordinate (expressed in arbitrary units). Moreover, as shown in FIG. 1b, the reflection spectrum of a uniform grating typically exhibits a central peak at the Bragg wavelength, and a plurality of secondary lobes. Said secondary lobes can be disadvantageous in some applications, for example when the Bragg grating is used to filter a channel (at a respective wavelength) in a multi-channel optical transmission system. In this case, in fact, the secondary lobes of the reflection spectrum introduce an undesired attenuation into the transmission channels adjacent that to be filtered.
In apodised gratings, the pattern of the refractive index n(z) is of the type qualitatively shown in FIG. 2a (wherein n is normalised to 1 and z is expressed in arbitrary units). As it can be noted, the term xcex94n(z) is suitably modulated in order to have a reduction of the above-mentioned secondary lobes. A typical pattern of the reflection spectrum of an apodised grating is illustrated in FIG. 2b. The reduction of the secondary lobes around the main reflection peak is evident. Such a grating can thus be advantageously used for filtering a channel in a multi-channel system, reducing the above-mentioned problem of the attenuation of the channels adjacent that filtered.
In chirped gratings, either of the terms n0(z) and xcex9(z) is variable. Due to this variability, and due to the fact thatxe2x80x94according to what said beforexe2x80x94the Bragg wavelength is proportional to the product between n0(z) and xcex9, said gratings have a broader reflection band with respect to uniform gratings. FIGS. 3a, 3b and 3c respectively show the qualitative pattern of the refractive index in the case the term n0(z) is modulated, and in the case the term xcex9(z) is modulated (for example, with a continuous variation from about 500 nm to about 502 nm), as well as the typical reflection spectrum of a chirped grating. As it can be noted from the spectrum of FIG. 3c, the reflection peak is significantly broadened. Such a grating can thus be used as wideband reflection filter or, more typically, as a device for compensating the chromatic dispersion. If, in addition, also the term xcex94n(z) is modulated, the grating will be of the apodised chirped type.
The international patent application WO 00/29884 in the name of The University of Sidney describes an arrangement for writing a grating in a photosensitive optical fibre. A UV light from a UV light source impinges on an aperture mask which is in the form of a series of spaced apart lines, the lines being opaque to UV lights. The UV light passes between the gaps in the aperture mask and is imaged by a lens having a focal length. The fibre is placed near the focal point and the aperture mask is imaged on the photosensitive fibre so as to form a grating structure. The position of the fibre can be moved forward and backwards so as to alter the periodicity of the grating (i.e. the image size)
Various techniques for writing an apodised Bragg grating are known. According to these techniques, the fibre is exposed to suitably shaped UV interference fringes so as to obtain a corresponding pattern of the refractive index, in particular of the local envelope xcex94n(z).
The current techniques can substantially be divided into two categories: that of interferometric techniques, and that of phase masks.
Interferometric techniques essentially consist in splitting a UV beam into two components, and in causing them to impinge onto the fibre at a predetermined relative angle so as to generate the interference fringes that induce the desired variation of the refractive index. These techniques are very versatile because by changing the relative angle between the two components, it is possible to change the grating parameters, in particular its period.
Nevertheless, interferometric techniques are not very suitable for mass production since the writing set-up is particularly sensitive to external agents (temperature, vibrations, etc.); thus, it requires several interventions for re-aligning the components. Therefore, their application is essentially limited to the research field.
Examples of interferometric techniques are provided, for example, in the article by Frxc3x6hlich and Kashyap, xe2x80x9cTwo methods of apodisation of fibre-Bragg-gratingsxe2x80x9d, Optics Communications, 157, 1998, 273-281.
Phase-mask techniques are generally deemed as more suitable for large-scale production due to a high repeatability, a lower susceptibility to mechanical vibrations, and to the fact that UW beams require a shorter coherence length.
A phase mask is a quartz substrate on a face of which there is, along a main direction, a series of rectilinear ridges parallel to one another, which define, in section, a substantially square-wave pattern. Typically, said ridges are equally spaced and of equal height in the case of a uniform mask, with variable pitch in the case of a chirped mask, and with variable height in the case of an apodised mask.
For writing the grating, the phase mask is usually arranged in front of the portion of fibre concerned, oriented so that its main direction (as defined above) is parallel to the fibre axis. When passed through by the UV radiation, the phase mask generates, at the output, interference fringes with a substantially sinusoidal pattern and with a period xcex9 equal to half the period xcex9m of the ridges of the mask itself. More in detail, at the output of the phase mask there are different orders associated to respective angles according to the following relation:                               sin          ⁢                      xe2x80x83                    ⁢                      θ            m                          =                  m          ⁢                      xe2x80x83                    ⁢                      λ            Λ                                              (        3        )            
The above-mentioned fringes are generated starting from the +1 and xe2x88x921 orders (values +1 and xe2x88x921 of m), whereas the other orders, in particular the zero order, are undesired since they tend to lower the visibility v of the fringes. The latter is defined, in first approximation, by the following relation:                     v        =                                            I              max                        -                          I              min                                                          I              max                        +                          I              min                                                          (        4        )            
where Imax and Imin are the peak and valley intensity of the fringes, respectively.
In general, phase masks are designed in such a way as to reduce (typically between about 1% and 3%) the transmitted zero order, and maximise (typically between about 30% and 40%) the amount of light in +1 and xe2x88x921 orders.
The article by J. Hxc3xcbner, M. Svalgaard, L. G. Nielsen and M. Kristensen, xe2x80x9cPhenomenological Model of UV-induced Bragg Grating Growth in Germanosilicate Fibersxe2x80x9d, SPIE Vol. 2998, presents a model for the growth behaviour of Bragg gratings in a germanosilicate fibre when the phase-mask technique is used. The authors of said article have carried out experiments by illuminating the fibre with an excimer laser (of the ArF or KrF type), or a frequency doubled argon ion laser (FRED). In the case of excimer laser, the laser beam was shaped by two cylindrical lenses so as to have a section of a size equal to the length of the grating to be written; the fibre was arranged in a central position with respect to the beam, and the phase mask was arranged in contact with the fibre. In the case of FRED laser, the beam was shaped by a cylindrical lens on a line parallel to the fibre axis, and the phase mask was arranged at about 100 xcexcm from the fibre.
The international application WO 99/22256 relates to an optical grating fabrication apparatus comprising a phase mask for dividing an incident light beam into a plurality of diffracted beams; and a focusing arrangement for receiving light from the phase mask and converging at least two non-zero-order diffracted beams together so as to generate an interference region between the converged beams so that a grating structure can be impressed on an optical waveguide placed in the interference region; the phase mask and at least a part of the focusing arrangement being moveable with respect to one another so as to alter the angle of convergence of the converged beams.
A phase mask can be used for writing apodised gratings. Some examples of known techniques for writing apodised gratings through phase mask are described hereinafter.
A first technique consists in using apodised phase masks, which have a variable diffraction efficiency; in this technique, contrary to what generally happens, variations of intensity of the beams of xe2x88x921 and +1 orders are used for modulating, according to the desired apodisation profile, the visibility of the fringes along the fibre axis. The article by Albert et al., xe2x80x9cApodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiencyxe2x80x9d, Electronics Letters, Vol. 31, No. 3, 1995, p. 222-225, describes for example the creation of apodised gratings with said technique. In particular, said article proposes the production of apodised masks wherein the ridge depth is variable.
The Applicant believes that even though this technique allows a relatively high repeatability and a relatively easy actuation, it is very unflexible as relates to the obtainable apodisation profile, as the latter is fixed by the shape of the phase mask. Moreover, the Applicant has noted that apodised phase masks are very expensive, and more subject to damage than the other mask types.
A second technique, presented in the article by Malo et al., xe2x80x9cApodised in-fibre Bragg grating reflectors photoimprinted using a phase maskxe2x80x9d, Electronics Letters, Vol. 31, No. 3, p. 223-225, 1995, consists in creating an apodised grating through a double exposure that allows obtaining a substantially constant mean value of the refractive index. The first exposure is made with an amplitude mask with variable transmissivity along a main axis thereof according to the desired apodisation profile, and the subsequent exposure is made with a periodic phase mask together with a second amplitude mask with transmissivity complementary to the first one.
As regards to this technique, the Applicant believes thatxe2x80x94since two subsequent exposures are provided and since the increase in the refractive index as a function of the irradiating energy is not linearxe2x80x94the mean refractive index obtained at the end of the double exposure can turn out to be not very uniform along the fibre axis.
The international patent application WO98/08120 by PIRELLI CAVI E SISTEMI S.P.A. describes a technique defined xe2x80x9cContinuous Fiber Grating Techniquexe2x80x9d, wherein a fibre, exposed through a mask to a UV radiation periodically time modulated, is continuously translated along its axis (by a translation stage controlled by an interferometric system) so that subsequent exposures produce overlapped fringes. Causing a dithering of the writing position on the fibre, said technique allows creating an apodised grating.
Even though this technique exhibits a high flexibility degree with respect to the obtainable apodisation profiles, it requires an expensive apparatus.
The article by Cole et al., xe2x80x9cMoving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase maskxe2x80x9d, Electronic Letters, Vol. 31, No. 17, p. 1488-1489, 1995, describes a technique for writing apodised gratings that, besides providing for the scanning of a laser beam in parallel to the fibre axis, and its passage through a phase mask facing the fibre itself, also provides for a relative displacement between fibre and mask in parallel to the fibre axis. The speed of fibre-mask relative displacement is much slower than the scanning speed of the laser beam. By controlling said relative displacement, it is possible to obtain chirped, apodised and phase-shifted gratings.
U.S. Pat. No. 5,912,999 describes a method for writing relatively long gratings wherein the fibre is displaced longitudinally at a controlled speed with respect to the mask and, for creating apodised gratings, the illuminating laser beam is amplitude modulated.
As regards this technique, the Applicant believes that the modulation of the energy induces a modulation of the mean refractive index n0(z) and, consequently, the growth of undesired lobes at shorter wavelengths with respect to the central one.
EP 0684491 describes a method for writing Bragg gratings, wherein the fibre is illuminated with interference fringes generated by the passage of electromagnetic radiation through a phase mask oriented so as to have its diffractive elements inclined at a predetermined angle (typically, a right angle) with respect to the fibre axis. During the writing, the spacing between the phase mask and the fibre is progressively changed, by a piezoelectric, according to a ramp pattern. This relative movement, in the range of dozens of micrometers, is caused in order to reduce the sensitivity of the grating writing to the spacing between the phase mask and the fibre.
The Applicant notes that said method does not make any apodisation of the grating. Moreover, the Applicant notes that the displacements provided by said technique, that is to say, up to at most 50 xcexcm, are not sufficient for a significant modification of the fringe visibility. Thus, such a displacement only allows an averaging operation of the interference fringes.
The international patent application WO 00/02068 in the name of Corning Incorporated describes a system for making an optical filter in an optical waveguide medium, comprising a spatial filter that extends spatial coherence of a beam emitted from a source of actinic radiation, and a phase mask that converts the spatially coherent beam into two interfering beams that illuminate the optical waveguide medium within the extended spatial coherence of the beam for producing index modulations in the medium, the phase mask being spaced from the optical waveguide medium by a distance that separates peak intensities of the interfering beams along the optical waveguide medium and that levels combined intensities of the interfering beams within a range of overlap between the interfering beams along the optical waveguide medium.
The Applicant faced the problem of providing a technique for writing a Bragg grating in a waveguide, in particular an apodised Bragg grating, which should allow generating gratings with desired spectral characteristics (that is to say, gratings with a predetermined pattern of the refractive index) with a high process flexibility, high repeatability, and which should not require high-precision alignments or expensive equipment.
The Applicant has found a technique that meets the above requirements, comprising the steps of scanning an ultraviolet radiation beam along a photosensitive portion of the waveguide through a phase mask directly facing said waveguide portion and, during the scan, changing in a controlled way the mask-guide distance so as to obtain the desired pattern of the envelope of the refractive index. Moreover, by changing in a controlled way also the intensity of the beam, it is possible to obtain the desired pattern of the mean value of the refractive index.
In a first aspect thereof, the present invention relates to a method for writing a Bragg grating in a waveguide, comprising the steps of:
providing a photosensitive waveguide portion in a writing position;
generating an ultraviolet radiation beam; and
scanning said beam along a z axis of said photosensitive waveguide portion through a phase mask directly facing said photosensitive waveguide portion and adapted to generate interference fringes with a period xcex9(z); wherein, during said scan, the step of changing the distance between phase mask and photosensitive waveguide portion is carried out in order to obtain an envelope xcex94n(z) of the refractive index n(z) along said z axis.
The pattern of the refractive index n(z) to be obtained at the end of the writing process along a z axis of the waveguide, in particular the patterns of the mean local value n0(z), of the envelope xcex94n(z) and of the period xcex9(z) of the refractive index n(z) are advantageously predetermined before starting the process.
Preferably, the method also comprises the step, carried out during said scan, of changing the energy of the beam along said photosensitive waveguide portion so as to obtain said mean local value n0(z)
Preferably, said step of changing the energy of the beam along said photosensitive waveguide portion comprises applying a variable attenuation to said beam during said scan.
The method can further comprise the step of reducing the size of said beam during said scan.
Said step of reducing the size of said beam can comprise transmitting said beam through a slit of a predetermined size.
Said step of scanning said beam can comprise the step of translating said slit.
Alternatively, said step of scanning said beam can comprise the steps of deflecting said beam by a mirror and translating said mirror in parallel to said portion of photosensitive waveguide.
As a further possibility, said step of scanning said beam can comprise translating said photosensitive waveguide portion along said z axis.
The method can further comprise the step of focusing said beam on said portion of photosensitive waveguide.
Said step of changing the energy of the beam along said photosensitive waveguide portion can alternatively comprise changing the speed of said scan.
In a second aspect thereof, the present invention relates to an apparatus for writing a Bragg grating in a waveguide, preferably an optical fibre, comprising:
an emitter of an ultraviolet radiation beam;
support elements of said waveguide for arranging a photosensitive waveguide portion in a writing position along a path of said beam;
a phase mask arranged along said path in such a position as to directly face said photosensitive waveguide portion when the latter is in writing position; and
means for scanning the beam along said photosensitive waveguide portion through said phase mask;
and also comprising a moving device for changing the distance between said phase mask and said photosensitive waveguide portion during the scan of the beam.
Preferably, the apparatus comprises a device for controlling the intensity of the beam adapted to operate during the beam scan.
Said device for controlling the intensity of the beam can be an optical attenuator adapted to receive the beam from said emitter.
Preferably, said moving device comprises a first motorised translation stage.
The apparatus can also comprise a screen provided with a slit arranged along said path of the beam before said phase mask, said slit having a smaller size with respect to that of the section of said beam.
Said scanning means advantageously comprises a second motorised translation stage carrying said screen and having an orthogonal displacement direction with respect to said beam for arranging said slit in different points of the section of said beam.
Moreover, the apparatus can comprise a mirror for deviating said beam towards said photosensitive waveguide portion.
In this case, said scanning means preferably comprise a second motorised translation stage carrying said mirror and said screen and having a displacement direction parallel to said photosensitive waveguide portion.
The apparatus can advantageously comprise a control and processing unit adapted to control said scanning means, said moving device and/or a device for controlling the beam intensity, adapted to change said intensity during the scan of the beam.
Moreover, there can be at least one optical element for focusing said beam on said photosensitive waveguide portion.
Advantageously, the distance between said phase mask and said photosensitive waveguide portion is greater than 50 xcexcm, preferably comprised between 100 xcexcm and 1000 xcexcm, more preferably comprised between 100 xcexcm and 800 xcexcm.
The divergence of said beam is preferably comprised between 0.5 and 1.5 mrad.
Said phase mask can advantageously be a periodic mask, and said emitter can advantageously be an excimer laser of the KrF type.
Said scanning means can alternatively comprise a motorised translation stage adapted to translate said photosensitive waveguide portion and said phase mask.
Said device for controlling the intensity of the beam can alternatively be a driving circuit of said emitter.
Said at least one optical element can comprise a cylindrical lens or, alternatively, a concave mirror.